It is a really nice CSS property which help FrontEnd developers to blur easly a background image (so without using a ::before child).
How I can vote for ? This proprety must be enabled by default. It is enabled by Default at other browsers.

You should see a "Vote" button up top, in the "Details" section of headers. Feel free to use it!

Having said that: this bug isn't trivial to fix -- we only have backdrop-filter implemented for one of our many graphics backends -- WebRender -- which is why it's not enabled by default. Right now I think we'd like to add some fancy logic to enable it wherever WebRender is available, per comment 1, but that is also difficult to do. (But I believe someone's going to be looking into that soon, so this is indeed on our radar.)

Also: please be aware that we discourage "I vote for this" / "please fix this" bug-comments on Bugzilla -- they don't add information that's useful for fixing the bug (beyond "one person thinks we should do this" which obviously doesn't scale well to many such comments :)). We try to keep bug reports "lean" & focused on bug diagnosis & implementation strategy as much as possible, so whichever developer ends up working on the bug can narrow in on the relevant information quickly without having to skim past unrelated (if supportive) comments.

So: please don't be offended by this, but I'm going to mark comment 4 as "advocacy" (which simply autocollapses it to avoid distracting from eventual implementation discussion). And similarly, I'll mark this comment that I'm typing here as "obsolete" to autocollapse my own comment & avoid adding too much additional noise.